Field sequential color display systems have a long history, including a significant role in the early development of color television. For a variety of reasons, such systems never achieved substantial commercial success. Perhaps the main drawback was the lack of a fast, low cost color selection device. Indeed, early field sequential TV systems employed a mechanical color wheel for this task, imposing a practical limitation on the size of the picture tube that could be used. Larger sized displays were produced using projection techniques, but at a cost of significantly reduced image brightness.
The development of fast liquid crystal optical switches in recent years has focussed new attention on field sequential color display systems. Such systems are described, for example, in U.S. Pat. Nos. 3,781,465 to Ernstoff et al., 4,003,081 to Hilsum, and 4,295,093 to Middleton, and in "Liquid-crystal shutter changes monochrome TV images into color", Electronics, Mar. 24, 1981, pp. 81, 82. More recently, the development of a very fast-switching liquid crystal device known as a "pi-cell" has made it practical to provide less expensive high-resolution color displays for electronic instruments, such as oscilloscopes and logic analyzers. The pi-cell optical switch and an accompanying field sequential display are described in Bos et al., "A Liquid Crystal Optical Switching Device (.pi.-Cell)", 1983 S.I.D. Digest, pp. 30, 31; Vatne et al., "A High Resolution LC/CRT Color Display", 1983 S.I.D. Digest, pp. 28, 29; and Bos et al., "Design Considerations For The .pi.-Cell, A Fast Electro-Optical Switch", Proc. of the Third Int. Display Research Conf. (1983) pp. 478-481.
FIG. 1 illustrates the principles of operation of a limited color liquid crystal field sequential display. A cathode-ray tube (CRT) is employed which has a phosphor screen that emits light at a plurality of wavelengths, corresponding to two different spectral colors, and which operates in conjunction with the fast liquid crystal color switch. Although the two colors chosen most often are red and green, others can be used as well. In this example, yellow and cyan have been chosen.
When an image is displayed on the CRT screen, the yellow light and cyan light components of the image are plane polarized in directions orthogonal to each other by a color selective filter formed by a pair of pleochroic polarizers. The polarized light then traverses a liquid crystal device called a pi-cell which acts as a switchable half-wave retarder. The optic axis of the pi-cell is disposed at a 45 degree angle with respect to the absorption axes of both of the color selective linear polarizers. When the pi-cell is in its ON state, the light passes through the cell unaltered and strikes the final linear polarizer which is oriented to transmit only the polarization state of the cyan light. When the pi-cell is in its OFF state, it acts as a half-wave retarder of the color of light to which it is tuned. Hence, the polarization of light incident on the pi-cell is rotated by 90.degree., and only the yellow light is of the proper polarization to pass through the final linear polarizer.
By switching between the two polarization states while synchronously writing information on the CRT, a multi-color display is produced, the particular colors being determined by the relative intensity of the light produced by the CRT during each state of polarization. The range of obtainable colors for such a yellow-cyan system is illustrated on the chromaticity diagram shown in FIG. 2 and corresponds to the line joining the yellow and cyan points on the diagram.
In such field sequential liquid crystal systems, display brightness is a primary concern and considerable effort is devoted to matching the CRT phosphor emission peaks with the polarizers' transmissivities. Even so, inherent light losses created by polarization of the emitted light and the duty cycle of the liquid crystal cell reduces the theoretical maximum efficiency for transmitted white light to only 25%. In an actual physical implementation of such a system, the efficiency is generally in the range of 10-14%. The reduced brightness of such displays is largely compensated for by the inherent high contrast produced in high ambient light levels. Nevertheless, some applications require the use of a more expensive, high output CRT in order to obtain a sufficiently bright display.